Optical telecommunication systems rely on various types of multiplexing of optical signals onto a common transmission optical fiber to increase the amount of information that can be transmitted over the transmission optical fiber. The types of multiplexing used include wavelength division multiplexing (WDM), polarization division multiplexing (PDM), frequency division multiplexing (FDM), time division multiplexing (TDM), space division multiplexing (SDM) and mode division multiplexing (MDM).
Mode division multiplexing (MDM) involves combining optical signals (data streams) in different spatial modes supported by a multimode fiber (MMF) or a few-mode fiber (FMF). Conventional MDM systems employ FMFs that have a round core that support two-dimensional mode distributions along both radial and azimuthal directions. This requires the MDM system to have two-dimensional mode converters to couple the modes into and out of the transmission optical fiber. The complexity of these mode converters increases with the number of modes used, making the MDM system complex and expensive.